Recommendations of the Immunization Practices Advisory Committee (ACIP) Yellow Fever Vaccine
These revised Immunization Practices Advisory Committee (ACIP) recommendations on yellow fever vaccine update the previous recommendations (MMWR 1978:27:268-70). Changes have been made to clarify (1) the risks of acquiring yellow fever associated with travel to endemic areas; (2) the precautions necessary for immunization of special groups (infants, pregnant women); (3) procedures for immunization of persons with histories of possible egg allergy; and (4) simultaneous administration of other vaccines.
Yellow fever presently occurs only in Africa and South America. Two forms of yellow fever -- urban and jungle -- are epidemiologically distinguishable. Clinically and etiologically, they are identical (1,2).
Urban yellow fever is an epidemic viral disease of humans transmitted from infected to susceptible persons by a vector, the Aedes aegypti mosquito. In areas where Ae. aegypti has been eliminated or suppressed, urban yellow fever has disappeared; eradication of Ae. aegypti in a number of countries, notably Panama, Brazil, Ecuador, Peru, Bolivia, Paraguay, Uruguay, and Argentina, achieved in the early 1900s, led to the disappearance of urban yellow fever. The last Ae. aegypti- borne yellow fever epidemic occurred in Trinidad in 1954. However, periodic reinfestations of some countries have occurred in recent years, and other countries remain infested, including areas of Venezuela, Colombia, and Guiana, which border on the enzootic zone for jungle yellow fever. In West Africa, Ae. aegypti-transmitted epidemics continue to occur at frequent intervals and involve human populations in both towns and rural villages (3).
Jungle yellow fever is an enzootic viral disease transmitted among nonhuman primate hosts by a variety of mosquito vectors. It is currently observed only in forest-savannah zones of tropical Africa and in forested areas of South America, but occasionally extends into parts of Central America and the island of Trinidad. In South America, approximately 200-400 cases are recognized annually, mainly among persons with occupational exposures in forested areas; the disease is, however, believed to be greatly underreported. In Africa, epidemics involving forest mosquito vectors affect tens of thousands of persons at intervals of a few years, but few cases are officially reported. The disease may sometimes not be detected in an area for some years and then reappear. Delineation of affected areas depends on surveillance of animal reservoirs and vectors, accurate diagnosis, and prompt reporting of all cases. The jungle yellow fever cycle may be active but unrecognized in forested areas of countries within the yellow fever endemic zone (Figure 2).
Urban yellow fever can be prevented by eradicating Ae. aegypti mosquitoes or by suppressing their numbers to the point that they no longer perpetuate infection. At the present time, jungle yellow fever can most effectively be prevented in humans by immunization.
YELLOW FEVER VACCINE
Yellow fever vaccine * is a live, attenuated virus preparation made from the 17D yellow fever virus strain (4). The 17D vaccine has proven to be extremely safe and effective (5). The 17D strain is grown in chick embryo inoculated with a seed virus of a fixed-passage level. The vaccine is freeze-dried supernate of centrifuged embryo homogenate, packaged in one-dose and five-dose vials for domestic use.
Vaccine should be stored at temperatures between 5 C (41 F) and -30 C (-22 F) -- preferably frozen, below 0 C (32 F) -- until it is reconsitituted by the addition of diluent sterile, physiologic saline supplied by the manufacturer. Multiple dose vials of reconstituted vaccine should be held at 5 C-10 C (41 F-50 F); unused vaccine should be discarded within 1 hour after reconstitution.
Vaccination for international travel may be required under circumstances other than those specified herein. Some countries in Africa require evidence of vaccination from all entering travelers. Some countries may waive the requirements for travelers coming from noninfected areas and staying less than 2 weeks. These requirements may change, so all travelers should seek current information from health departments. Travel agencies, international airlines, and/or shipping lines should also have up-to-date information.
Some countries require an individual, even if only in transit, to have a valid International Certificate of Vaccination if he or she has been in countries either known or thought to harbor yellow fever virus. Such requirements may be strictly enforced, particularly for persons traveling from Africa or South America to Asia.
C. Primary immunization: For persons of all ages, a single
subcutaneous injection of 0.5 ml of reconstituted vaccine is used.
D. Booster doses: Yellow fever immunity following vaccination with 17D
strain virus persists for more than 10 years (7-9); the International Health Regulations do not require vaccination more often than every 10 years.
Reactions to 17D yellow fever vaccine are generally mild. Two percent to 5% of vaccinees have mild headaches, myalgia, low-grade fevers, or other minor symptoms 5-10 days after vaccination. Fewer than 0.2% curtail regular activities. Immediate hypersensitivity reactions, characterized by rash, urticaria, and/or asthma, are extremely uncommon (incidence less than 1/1,000,000) and occur principally in persons with histories of egg allergy. Although more than 34 million doses of vaccines have been distributed, only two cases of encephalitis temporally associated with vaccinations have been reported in the United States; in one fatal case, 17D virus was isolated from the brain.
PRECAUTIONS AND CONTRAINDICATIONS
If international travel regulations are the only reason to vaccinate a patient hypersensitive to eggs, efforts should be made to obtain a waiver. A physician's letter clearly stating the contraindication to vaccination has been acceptable to some governments. (Ideally, it should be written on letterhead stationery and bear the stamp used by health departments and official immunization centers to validate the International Certificates of Vaccination.) Under these conditions, it is also useful for the traveler to obtain specific and authoritative advice from the country or countries he or she plans to visit. Their embassies or consulates may be contacted. Subsequent waiver of requirements should be documented by appropriate letters.
If vaccination of an individual with a questionable history of egg hypersensitivity is considered essential because of a high risk of exposure, an intradermal test dose may be administered under close medical supervision. Specific directions for skin testing are found in the package insert.
SIMULTANEOUS ADMINISTRATION OF OTHER VACCINES
Determination of whether to administer yellow fever vaccine and other immunobiologics simultaneously should be made on the basis of convenience to the traveler in completing the desired immunizations before travel and on information regarding possible interference. The following will help guide these decisions.
Studies have shown that the serologic response to yellow fever vaccine is not inhibited by administration of certain other vaccines concurrently or at various intervals of a few days to 1 month. Measles, smallpox, and yellow fever vaccines have been administered in combination with full efficacy of each of the components; Bacillus Calmette Guerin (BCG) and yellow fever vaccines have been administered simultaneously without interference. Additionally, severity of reactions to vaccination was not amplified by concurrent administration of yellow fever and other live virus vaccines (10). If live virus vaccines are not given concurrently, 4 weeks should be allowed to elapse between sequential vaccinations.
Other studies have indicated that persons given yellow fever and cholera vaccines simultaneously or 1-3 weeks apart showed reduced antibody responses to both vaccines (11, 12). When feasible, cholera and yellow fever vaccines should be administered at a minimal interval of 3 weeks, unless time constraints preclude this. If the vaccines cannot be administered at least 3 weeks apart, they should be given simultaneously. There are no data on possible interference between yellow fever and typhoid, paratyphoid, typhus, hepatitis B, plague, rabies, or Japanese encephalitis vaccines.
A recently completed prospective study of persons given yellow fever vaccine and 5 cc of commercially available immune globulin revealed no alteration of the immunologic response to yellow fever vaccine when compared to controls (13).
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